As substrates for heat-sensitive recording materials, various sheet-like substrates such as paper and film-like substrates such as synthetic resin films are known. In general, sheet-like substrates such as paper have disadvantages in recording density and image quality although they are inexpensive. On the other hand, film-like substrates such as synthetic resin films provide recorded images of high density but, as compared with paper and the like having the same thickness, are poor in heat resistance and stiffness (so-called "nerve"). Therefore, the latter have disadvantages in that areas colored with high density at the time of heat recording are warped and readily deformed, and particularly in the case of thin films, troubles such as buckling and jamming with a heat-sensitive recording apparatus arise because they are limp. For this reason, it may be considered to use thick films. In this case, however, the problem arises in that the cost is excessively high. Accordingly, a laminate of an inexpensive paper having high stiffness with a thin film is usually used.
In bonding together the paper and the film, in general, an aqueous or solvent type adhesive is used. In any of the cases, in order to remove off water or the solvent after bonding, heating is needed. Since, however, synthetic paper and films are poor in heat resistance, heating is accompanied with heat shrinkage, deformation, strain and further problems such as curling and the like. For this reason, it has been proposed to use a reactive adhesive. In this case, however, problems arise in that long term aging is needed in the roll state in order to cure the adhesive, and further the pot life of the adhesive is short.
In the case where a thin film having a thickness of about 10 to 60 .mu.m is bonded together with a substrate such as paper, if the paper is not made sufficiently smooth, irregularities of the paper exert great influences on the film surface and, as a result, a heat-sensitive recording material using such paper as a substrate cannot provide a recorded image of high quality.